Inferred wind profiles and surface roughness during the Beagle 2 landing on Mars

Abstract

The Planetary Boundary Layer (PBL) on Mars is a dynamic region of the atmosphere that extends several kilometres above the surface. It is sensitive to solar heating during the day, generating turbulent circulations and large convective cells. The background wind is influenced by the large-scale winds and interactions with the surface. It is important to characterise this region of the atmosphere to better understand atmospheric circulations and to verify atmospheric models.

Wind profiles between the surface and to several kilometres above the PBL are generally lacking for Mars. However these can be inferred by analysing the displacement of jettisoned hardware at landing sites. This has been done for a variety of landers in the past to help verify atmospheric models and investigate the Martian atmosphere. Here we determine the wind speed and direction during the descent of Beagle 2 and also investigate the surface.

1. Introduction

Beagle 2 landed in Isidis Planitia basin in the early afternoon, during the northern hemisphere winter at Ls=322°. The region where Beagle 2 landed is relatively flat suggesting the lack of local slope winds. However around sides of the basin are steep slopes that are susceptible to slope winds, e.g. Chojnacki et al. (2019) which drive the circulation at the landing site (Rafkin et al., 2004). Atmospheric modelling data from the Mars Climate Database (MCD) indicates a strong influence on the winds from the thermal tide (Bingham et al., 2004). Results from the Mars Regional Atmospheric Modeling System (MRAMS) (Rafkin et al., 2004) indicate slope-wind driven circulations modulated by the large-scale circulations.

2. Method

The wind profiles at the landing site are inferred based on the location of hardware identified in HiRISE images of the Beagle 2 landing site (Bridges et al., 2017; Clemmet et al., 2017). We use a trajectory model (Paton et al., 2018) to calculate the impact locations of Beagle 2’s heat shield, backshell-pilot parachute, main parachute and the airbags. The winds in the trajectory model are varied until the end points in the model match the actual locations of Beagle 2 hardware.

The modelling of the airbag trajectory, as it bounces across the surface, can tell us something about the surface roughness. For a smooth surface the airbags bounce in a straight line while for a rough surface the airbags can be deflected from its path. By comparing the inferred wind profile to an atmospheric model wind profile we can constrain the surface roughness. The surface roughness parameter used in the model is the maximum metre-scale slope angle.

The elevation of the Beagle 2 landing site is established using a Digital Terrain Model (DTM) together with MOLA elevation data. The DTM is also be used to estimate the flatness over the distance bounced by the airbags (about 100 m). Noise in the HiRISE images, used to make the DTM, makes it difficult to estimate the metre-scale roughness which is relevant to the airbag’s traverse across the surface.

3. Results 

A three-layer wind profile was inferred for the Beagle 2 landing. A good match between the inferred and MCD wind speed profiles was found when the metre-scale surface roughness in the trajectory model is comparable to that at the Pathfinder landing site. There is a good match with high-resolution results from MRAMS. The inferred, MRAMS and MCD wind profiles indicate high-speed winds of about 15 m s^-1 above 3 to 4 km. Below these altitudes the wind speed drops down to 5 m s^-1. Below ~3 km altitude the wind speed is relatively uniform, typical of a well mixed PBL.  

The inferred wind direction above 3 km altitude matches the MCD quite well with a wind approximately from the east. There is a significant discrepancy between the inferred wind direction and the MCD near the surface. This may be due to large-scale convective structures that at the Beagle 2 landing site.

Comparisons between an inferred wind speed profile and atmospheric model data from the time of the Perseverance landing (Paton et al., 2024), requires high resolution atmospheric modelling for a good match. At the Beagle 2 landing site, which DTM data indicates is relatively flat, both the MCD and MRAMS atmospheric models appear to agree with the inferred wind speed profile.

4. Conclusions

The lower layers of the inferred wind profiles at the Beagle 2 landing site appears to be broadly consistent with predictions. These winds are primarily driven by the slope-driven winds occurring at the edges of Isidis Planitia. The upper layer of the wind speed profile is consistent with the large scale flow above the PBL when surface roughness is included in the trajectory model for the airbags.

References

Bingham, S.J., et al., 2004, Environmental predictions for the Beagle 2 lander, based on GCM climate simulations, Planetary and Space Science, 52, 259–269

Bridges, J. C. et al., 2017, Identification of the Beagle 2 lander on Mars., Royal Society Open Science, 4

Chojnacki, M., et al., 2019., Boundary condition controls on the high-sand-flux regions of Mars, Geology, 47, 427–430

Clemmet, J., et al., 2017, Beagle 2 on Mars - the Discovery Assessed, Journal of the British Interplanetary Society, 70, 262–277

Paton, M. D., Harri, et al., 2018, Measurement of Martian boundary layer winds by the displacement of jettisoned lander hardware, Icarus, 309, 345-362

Paton, M. D., et al., 2024, Inferred wind speed and direction during the descent and landing of Perseverance on Mars. Icarus, 415

Rafkin, S. C. R., et al., 2004, Meteorological predictions for the Beagle 2 mission to Mars, Geophysical Research Letters, 31